Gold nanostars are multi-branched nanoparticles with tip structures. Nanostars have excellent photoelectric properties, which make them able to be used in a variety of optoelectronics devices. Moreover, these stars have good biocompatibility and low toxicity, which opens broad application prospect in the biomedical field. Gold nanostars with admirable optical as well as thermal properties, are thought as a good candidate in cancer treatment that is a hot research topic in recent years. Gold nanostars with different branch-lengths were prepared by the photo-assisted method, and the effect of light was well studied in relation with gold nanostar branch-length. In the solution system, HEPES was used as the reducing agent, stable agent and shape-inducing agent. Under light irradiation, a certain amount of chloroauric acid solution (HAuCl4) was added to the HEPES solution. After a period of time, gold nanostars were prepared. Different wavelengths of irradiating light were selected in this experiment. The wavelength has different effects on the growth of branches associated with gold nanostars. The transmission electron microscope and the ultraviolet-visible-near infrared spectrophotometer were used to analyze the morphology and absorption spectra of gold nanostars. Meanwhile, a nano-measurer software was used to determine branch-lengths of gold nanostars under light irradiation of different wavelengths. The results indicate that the branches of the nanostars under irradiation were shorter than those of nanostars without irradiation. Different branch lengths correspond to different irradiation wavelengths. Based on these results, the physical process of shortening nanostars branches was analyzed, and a theoretical model of changing branch-length in the process of light-induced nanostars growth was proposed. The model indicates that there are two steps when the branch-length is changing. Firstly, the branch-length grows longer with the overall growth of the nanostar. Secondly, the nanostar becomes shorter because of the insatiability of HEPES molecules that are adsorbed on the nanostar surface with the increasing solution temperature. Through a photothermal measurement, a xenon lamp (wavelength 670 nm) was used as a light source to measure the temperature change within 30 min, and then the photothermal conversion efficiency of the gold nanostars was calculated. The results show that the branch-length of gold nanostars can be precisely controlled by light irradiation with slight variation in wavelength. The photothermal conversion efficiency of gold nanostars can also be regulated.